The need for the present invention results from the inability of prior art devices to keep pace with recent developments in electronic circuitry and their methods of manufacture. Developments in solid state electronics and materials led to a quantum leap in the use of printed circuit boards (PC boards), ceramic substrates (substrates) and other similar items having conductive surfaces formed thereon and wherein it is necessary to establish continuity of the electrically conducting paths or traces together with the absence of undesired shorts. The PC boards or substrates, which typically are 1/32 of an inch to 1/8 of an inch thick, are composed of a phenolic plastic, glass epoxy, ceramic or other similar electrically insulating material, and range from postage stamp size up to several square feet. Bonded to the outer surfaces (and sometimes sandwiched between layers of insulation) are very thin coatings of copper, silver, gold or other metallic foils which act as conducting pathways, runs or traces. These paths are formed by patterns of very narrow lines of conducting material deposited or otherwise formed on the insulating layer of the PC board or substrate. At the end of each run is an enlarged area known as a pad which is used to connect either a component to the board or to make an external electrical connection to the board.
During the manufacture or subsequent handling of PC boards, substrates and the like, defects such as discontinuities (cracks) in a circuit pathway or run, or an unwanted continuity (short) between adjacent runs, may develop. Once components have been attached to the pads on printed circuit boards (usually by soldering) it is much more difficult and expensive to find and repair any such shorts and discontinuities which might exist. To deal with this problem, equipment was developed for automatically testing printed circuit boards and substrates for shorts and discontinuities before components were attached to the boards. This equipment works by applying to the various pads on a printed circuit board or ceramic substrate, voltages or other test signals, and then detecting these voltages or other signals at other specified pads on the board. With a voltage applied to a particular pad, all the other pads which should be connected via conducting pathways or traces to this first pad, should have a voltage present. Those which are not by design to have been electrically interconnected via pathways or traces, should not have a voltage present on them. By contacting many or all of the paths simultaneously, and using high speed electronic switching devices to connect and measure the respective testing voltages, testing of the boards could be accomplished relatively quickly.
The traditional device used to contact all of the pads of a printed circuit board or substrate simultaneously, is called a "bed of nails" fixture. As the name implies, this fixture consists of a frame with dozens or sometimes hundreds (maybe thousands) of small, nail-like contact points. Each contact has a wire extending from it to a machine which contains a test voltage source and measurement equipment via suitable interconnecting switches. To insure that each contact point makes good electrical contact to the proper pad, the contacts are usually gold plated, and the frames undergo precision manufacturing. Making such a "bed of nails" fixture is quite expensive and may take several weeks or months. As the number of contact points increases, the amount of pressure applied to the fixture and circuit board must also increase to assure good contact. This requirement itself may cause cracking of the boards or damage to the pad thus complicating the problem the equipment was originally designed to detect. Lastly, each type or size of circuit board to be tested requires its own particular "bed of nails" fixture.
Further increases in the complexity and decreases in the size of electronic circuit boards resulted in greater numbers of smaller sized traces or conductive pathways and pads being fitted onto a given area. These smaller, more fragile, traces and pads are more susceptible to shorts and discontinuities (cracks). Fixtures for automatically testing PC boards and substrates in this size range, required contact pins which are smaller (thinner) and more closely spaced. This has resulted in more expensive testing fixtures which are more delicate and thus more susceptible to damage from handling and use. This problem is particularly severe with respect to ceramic substrates and chip carrier assemblies presently being produced and used. On a ceramic substrate or chip carrier of this size range, there may be dozens or even hundreds of pads each of which can be as small as 20 mils (thousandths of an inch) and spaced apart by a comparable distance, namely 20 mils. The inability of the standard test fixtures to work with ceramic substrates or chip carriers resulted in the development of a specialized fixture called a probe card. The probe card uses very fine bent wires instead of pins to contact the pads on a substrate or chip carrier being tested. The probe card fits around the substrate or chip carrier in such a manner that the fine wires make contact with the substrate or chip carrier pads. Like the larger "bed of nails" fixtures, the probe cards just be precision manufactured, are expensive, are susceptible to handling damage, must be specifically designed for a particular type of ceramic substrate or chip carrier and require a very substantial lead time on the order of serveral weeks or even months to manufacture.